A conventional hand lay up molding method and a spray up molding method have been substituted by a method in which a sheet molding compound (SMC) material or bulk molding compound (BMC) material respectively containing as a main component an unsaturated polyester resin is used since the molding work of this method is simple, and these materials such as SMC and BMC have been used for a bath tub and the like.
However, these materials containing as a main component an unsaturated polyester resin exhibit a problem in that they were inferior with regard to hot water resistance and durability since the materials hydrolyzed under alkaline condition.
In order to improve this problem, various methods were suggested.
For example, Japanese Patent Application Publication (JP-B) No. 5-13899 discloses a method for producing a natural grain-like article wherein (i) 100 parts by weight of a "monomer A" containing 20% by weight or more of a polyfunctional (meth)acrylate, (ii) 50 to 300 parts by weight of a "methacryl-based resin powder B" having a grit of 20 mesh or less and an average molecular weight of 50,000 to 200,000 and being derived from a monomer containing methyacrylates as an essential component, (iii) 0.5 to 2 parts by weight of a "curing agent C" for the monomer A, (iv) 100 to 500 parts by weight of an "inorganic filler D", and optionally, (v) a coupling agent, a coloring agent and a reinforcing agent are mixed, kneaded and aged with heating at room temperature or at a temperature lower than the curing temperature to obtain a molding material which is in clay form or solid form at room temperature. The produced molding material is molded and cured under pressure with heating.
Japanese Patent Application Laid-Open (JP-A) No. 5-32720 discloses an acrylic resin premix comprising (i) 20 to 60% by weight of an unsaturated monomer containing methyl methacrylate as a main component, (ii) 80 to 40% by weight of an inorganic powder, and (iii) 1 to 25% by weight of a resin fine particle which has a degree of swelling in methyl methacrylate of 3 to 15 times and is insoluble in methyl methacrylate.
Japanese Patent Application Laid-Open (JP-A) No. 7-188505 discloses an acrylic resin premix comprising, (i) based 100 parts by weight of an "acrylic syrup A" comprising as main components an unsaturated monomer essentially composed of methyl methacrylate and a polymer essentially composed of a polymer of methyl methacrylate, (ii) 180 to 260 parts by weight of an "inorganic powder B" and (iii) 60 to 140 parts by weight of a "resin particle C" containing an inorganic powder, wherein the resin particle C is insoluble in methyl methacrylate, and shows an oil absorption in methyl methacrylate after one day of 40 to 100% by weight and a degree of swelling in methyl methacrylate of 2 times or less.
However, in the method of Japanese Patent Application Publication (JP-B) No. 5-13899, the methacryl-based resin powder having an average molecular weight of 50,000 to 200,000 is used as a resin component and, therefore, there are problems regarding handling properties. Stickiness of the surface after the kneading and aging is remarkable, shape stability after the kneading is not sufficient and the kneaded resin flows or is deformed when only left.
Further, all of the premixes described in Japanese Patent Application Laid-Open (JP-A) No. 5-32720 and Japanese Patent Application Laid-Open (JP-A) No. 7-188505 contain the unsaturated monomer component essentially composed of methyl methacrylate, can not suppress evaporation of methyl methacrylate, are deficient in stability when stored or left, and further, have a bad odor. Therefore, improvement has been desired not only in view of handling properties but also in view of working environments.